A new method of alternating shooting of two different seismic sources for deep geophysical surveys

In the latest geophysical survey crossing the Ninety East Ridge of the Indian Ocean, a new method was employed to perform proportional double seismic source excitation and synchronously receive signals from the sea surface and the seabed. The two seismic sources used for excitation were two sets of gun arrays with different energies and dominant frequencies, a G gun array and a Bolt gun array. The G gun array consisted of 3 G.II guns with a total capacity of 450 in3 and a dominant frequency of 20–100 Hz. The Bolt gun array consisted of 4 Bolt 1500LL air guns with a total capacity of 6000 in3 and a dominant frequency of 10–40 Hz. The seismic receiving system comprised a 480-channel seismic streamer towed from the sea surface and 21 ocean bottom seismometers (OBS). During offshore operations, the integrated navigation system produced equidistant trigger signals at an interval of 50 m. The trigger signals were distributed to the G gun array and Bolt gun array at a ratio of 3:1 after passing through a pulse signal proportional distributor. The two sets of gun arrays fired alternatingly at a given ratio. The receiving equipment on the sea surface and seabed simultaneously received the seismic signals excited by the two sets of gun arrays. After targeted data processing, in addition to the seismic profile generated by the conventional G gun seismic source, the deep seismic profile generated by the Bolt gun seismic source and the survey profile of the active-source OBS were obtained simultaneously. The penetration depths of the three sets of profiles reach 2 km, 6 km, and 30 km, respectively, greatly improving the efficiency of offshore deep-sea seismic surveys.


Methods
Air gun seismic source. In the MCS survey, a G gun array composed of three 150 in 3 G.II guns was used, with the three air guns arranged in a "Δ" shape, which is also known as a delta seismic source. The distance between the three air guns was 0.9 m, the seismic source was 6 m from the water surface, and the total capacity of the air guns was 450 in 3 . During the active-source OBS survey, a Bolt gun array composed of 4 Bolt 1500LL air guns was used, with a total capacity of 6000 in 3 . The distance between the air guns and the water surface was 10 m. The air gun spacing is shown in Fig. 2. The air-gun array wavelet simulation can effectively and theoretically simulate the far-field wavelet morphology and spectrum characteristics of air-gun arraywith different combination methods and provide a theoretical basis for the design and optimization of air gun seismic source parameters during field seismic surveys and acquisition construction 13 . Figure 3 shows the far-field wavelet theory simulation results of the air-gun array used for this voyage. This figure clearly shows that the delta seismic source composed of G.II guns has a high dominant frequency and a wide effective frequency band, which can satisfy the requirements of shallow highresolution reflection seismic surveys for seismic source wavelets. The Bolt gun is a large-capacity air gun seismic source with a capacity of up to 6000 in 3 . It can excite strong seismic waves with a low dominant frequency and a high energy at the low-frequency end of the far-field wavelet spectrum, which increases the exploration depth of seismic surveys and can yield more information on tectonic structures at depth.

MCS and OBS signal acquisition.
The Seal 428 acquisition system was used for MCS acquisition. The total number of channels of seismic cables was 480, and the spacing between channels was 6.25 m. Three types of OBS were employed to receive signals from the gun arrays, including the GeoPro SeidisVI made in Germany, MicrOBS made in France, and PanGui OBS developed by South University of Science and Technology of China. The MCS cables and OBS system simultaneously received signals from the two air gun sources. Figure 4 shows the signals of the two air gun arrays recorded by the MCS system.  www.nature.com/scientificreports/ Working process. The first step was to place the OBS cloth on the seabed, followed by laying the MCS cable.
The Bolt gun and delta seismic sources were then deployed into the water and fixed to the left and right sides of the stern, respectively. The integrated navigation software SeaproNav triggered the firing of the guns according to a shot spacing of 50 m. The parameters of the seismic profiles are shown in Table 1.
A signal distributor was specially designed to send the received trigger signals to two Bigshot air gun controllers at a ratio of 3:1, and then the two air gun controllers excited the delta seismic source and the Bolt gun seismic source (Fig. 5).
Data processing MCS data processing. Compared with routine MCS data, due to the use of two completely different AIR GUN ARRAY seismic sources firing alternatingly, the original seismic shot data set had problems, such as an uneven energy and inconsistent waveform take-off times for different shot energies, as shown in Figs. 6 and 7. These problems had to be suppressed in the pretreatment stage. In addition, the very-low-frequency interference on the seismic data of this voyage was relatively well developed, and the dominant energy was mostly approximately 1-4 Hz, which was easy to suppress while processing. The prevailing lateral reflection waves had a greater impact on the offset and needed to be suppressed. The air gun seismic source wavelet produced an evident bub-   www.nature.com/scientificreports/ ble effect, which interfered with shallow strata imaging and easily formed a structural illusion. Multiples of the seabed were relatively well developed, which was also the main factor affecting the signal-to-noise ratio of the data in this area. The above types of interference waves require targeted processing. Figure 8 shows the processing flows of the MCS data. The key processing techniques included prestack gather purification processing, combined deconvolution processing 14 , multidomain combined multiple attenuation processing 15,16 , and curved ray Kirchhoff prestack time migration imaging 17 . Finally, according to the original MCS data collected by different alternatingly excited seismic sources, two sets of seismic imaging profiles were obtained. Figures 9 and 10 show two different seismic profiles and spectrum analysis results. The seismic profile obtained by the G gun source has a higher frequency band and a higher resolution (see Fig. 9a), while the seismic profile obtained by the Bolt gun source has a deeper detection depth and can receive reflected signals from the Moho (see Fig. 9b).
OBS data processing. The processing of the longitudinal wave data was determined according to the practice of Chinese scientists conducting active-source OBS surveys in the Bohai Sea and Huanghai Sea 10,18,19 , the East China Sea 20,21 , the South China Sea [22][23][24][25] , and the Southwest Indian Ocean 26 and by referring to the OBS imaging methods employed for the Emperor Seamount Chain and Louisville Ridge in the Pacific Ocean and the Whale Ridge in the Atlantic Ocean [27][28][29] , as shown in Fig. 11. The workflow primarily included the following aspects: OBS data decoding and truncation: the precise time and position information of the excitation point is the key for active-source OBS data truncation preprocessing 16,24,26 . In this survey process, the precision gun controller triggered the AIR GUN ARRAY and recorded the precise firing time, and the position of the excitation point was the ship-borne GPS position. Therefore, the ship-borne GPS position had to be corrected to the position www.nature.com/scientificreports/ of the center of the AIR GUN ARRAY (the two were 40 m apart), and the Ukooa control file required for data truncation was obtained by combining the position information with the accurate time information. Next, for the two different types of raw OBS data, with the OBS drop point treated as the landing position, the data from the Chinese OBS and German OBS devices were decoded and truncated by using format conversion programs to obtain internationally accepted single-station common receiver gather (CRG) data in the SEGY format. OBS landing position and time corrections: Inshore surface unhooking and free fall dropping are common dropping methods for OBS surveys 30 . Due to marine environmental factors, such as sea breezes and ocean currents, the location of the OBS landing point obviously deviates from the actual drop point. Additionally, the quartz crystal oscillator used for OBS data collection causes clock drift due to factors such as temperature and humidity [30][31][32][33] . These drifts in position and time cause large errors in seismic wave travel time pickup 25,31,34 . The search method and the damped least square method were employed to correct the OBS landing point position and time when the direct water wave travel time was used. The specific methods were as follows: ① Pick up the direct water wave travel time information in the SEGY format as the actual arrival time, use the accurate multibeam water depth grid data as the constraint, select the multibeam grid node in the range of 2 km × 2 km with the dropping point as the center, and calculate the theoretical direct water wave arrival time. ② When comparing    www.nature.com/scientificreports/ has a relatively strong dependence on the initial model. Therefore, an accurate initial model with better control of the shallow strata helps reduce the forward modeling time and quickly obtain the optimal crustal structure model 36 . For this survey, the survey line was arranged to cross the Central Indian Ocean Basin, Ninety East Ridge and the Wharton Basin from west to east. The shallow sedimentary layer had a thickness of 100-800 m, and the sedimentary base fluctuated greatly. Using only the PsP seismic phase would have increased the uncertainty of the shallow stratum results, interfered with the effective fitting of the Pg seismic phase at the near offset distance of the OBS station, and caused the simulation to deviate from the true Moho surface depth and structure of the magma underplating body. Therefore, an initial model was constructed according to the multibeam central beam and MCS data on board, and this model could well restrict the morphology of the seabed and sedimentary substrate and the velocity distribution of the sedimentary layer. Comprehensive analysis was performed on the seismic phase characteristics of different stations along the survey line, and the Rayinvr software package was employed for ray tracing and forward modeling calculations 35 . As a result, the two-dimensional velocity structure model below the survey line was obtained on the top of the Ninety East Ridge, as shown in Fig. 12. This can be compared with the 17°S section on the Ninety East Ridge 37 .

Conclusion
In this geophysical cross-section survey in the Indian Ocean, a new method was designed and implemented to proportionally excite different seismic sources and synchronously receive signals from the sea surface and seabed. Three sets of seismic profiles with completely different resolutions and penetration depths were obtained through one firing operation, greatly improving the efficiency of deep seismic surveys. Compared with the conventional method, the new method provided in this article can obtain three different resolution stratigraphic profiles through one shot. The G gun seismic profile can provide high-resolution stratigraphic information above the bedrock. Bolt gun seismic profiles can provide lower resolution stratigraphic information above the bedrock and information of Moho surface. And the velocity structure profile of OBS can characterize the crustal structure above the Moho surface. Figure 11. OBS data processing process. www.nature.com/scientificreports/ Unlike traditional seismic profiles, the seismic profiles obtained by this method have a "take-off time" phenomenon due to trigger delays between different air gun, and energy imbalance due to different gas gun capacities. These issues need to be solved before conventional seismic data processing.
Similarly, if the seismic source was replaced by an electric spark seismic source and an air gun seismic source, a shallow high-resolution seismic profile and a seismic profile with a deeper detection depth but a relatively low resolution could be simultaneously obtained.

Data availability
The datasets generated and analyzed during the current study are not publicly available because data acquisition was supported by a project from the Ministry of Natural Resources. We do not have permission to share data, but more information is available from the corresponding author on reasonable request. www.nature.com/scientificreports/